1. Field
Example embodiments relate to semiconductor devices and methods of fabricating the same, and more particularly, to high electron mobility transistors (HEMTs) and methods of fabricating the same.
2. Description of the Related Art
High electron mobility transistors (HEMTs) include semiconductors with different band gaps and/or polarizabilities bonded together. In the HEMT, a semiconductor having a larger band gap functions as a donor. A two-dimensional electron gas (2DEG) is formed in a semiconductor having a smaller band gap by the semiconductor having a larger band gap.
A 2DEG may be used as a channel of a HEMT. A HEMT may be a kind of power transistor with a high breakdown-voltage and improved electron carrier mobility. The HEMT may include a semiconductor having a wide band gap, for example, a compound semiconductor. Therefore, the HEMT may have a high breakdown voltage.
The 2DEG may be induced by performing an n-doping process on a material having a large band gap or by using a material having polarization. The HEMT may include a channel without applied bias, and thus, may be normally on and operate in a depletion mode. During a turning-off operation of a HEMT, the 2DEG between a gate and a drain may be removed (e.g., by an applied voltage) and space charges remain. An electric field may concentrate on the gate due to the space charges. The breakdown voltage of the HEMT may be reduced due to the concentration of the electric field on the gate. In order to reduce a space charge electric field at the gate, a region performing the same function as that of a lightly doped drain (LDD) (hereinafter, referred to as an LDD region) may be induced in the 2DEG channel of the HEMT.
An LDD region may also lead to the formation of an enhancement mode HEMT. In considering simplification of circuits, a HEMT operating in an enhancement mode (hereinafter, referred to as E-mode) may be desired. The E-mode may be realized by removing at least a portion of a channel (e.g., electron density in the channel) located under a gate.
During the fabrication processes of the HEMT, the 2DEG channel with a relatively higher concentration of electrons may be induced first, and then, the concentration of electrons on a portion of the 2DEG channel, which corresponds to the LDD region, may be reduced. The 2DEG may be removed at a portion where the gate will be formed in order to form the E-mode HEMT. The electron concentration in the 2DEG channel may be reduced by etching an upper layer formed on the 2DEG channel and/or oxidizing the upper layer in order to form the LDD region. However, it may be difficult to control these methods for reducing the electron concentration when compared to an implant process that is used to form a silicon metal-on-semiconductor (MOS) transistor. Therefore, it may be difficult to form the LDD region having an appropriate electron concentration, and a reproducibility of the LDD region may be degraded. Thus, operations of a HEMT may not be reliable.